Partially correct. An aircraft requires two pilots by type certification or by the rules it's being operated under. With one pilot under the hood, a 172 now requires two pilots. The safety pilot could log PIC. if ACTING as PIC and this is agreed upon before the flight; otherwise it's SIC. Another caveat would occur in say a C172RG if the safety pilot did not have an HP/complex endorsement. In that case he MUST log SIC. So you can in fact have SIC in a 172. And in all my years of flight school ownership - looking at countless logbooks - that's the way I've seen it the most: SIC.